Caption: Oil on water. A swirl of coloured oil (orange) on water, showing the immiscibility of the two liquids. The vortex of oil forms a film on the water surface which is concentrated towards the centre; at the outer edges, the water coloured green can be seen. Oil and water are mutually insoluble, so will not mix without the addition of an agent (such as a detergent) that breaks the surface tension of both. Despite any amount of agitation, the oil and water will always separate out at rest.